Film reciprocity error is a problem inherent with most modern films, particularly the instant developing type films currently in use. The problem results in the film's lessened sensitivity to light exposure at the longer exposure times. For example, a film requiring ten millisecond exposure at a fixed shutter opening size will require 20 millisecond exposure at a lessened light intensity. At these light levels, the reciprocity error is minimal. However, going from ten milliseconds exposure to a ten-fold light level reduction requires more than ten times the length of exposure. This can be compensated by longer exposure time or by increasing the shutter size to allow the additional amount of light into the camera.
Empirical determination of reciprocity error has shown it to be of the following form: EQU T1/T0=(AT.sub.0).sup.1/B
where
T1=Actual shutter speed. PA1 T0=Uncorrected shutter speed. PA1 A=Constant used to describe shutter speed at which correction starts. PA1 B=constant used to describe amount of correction required.
One method of compensating for reciprocity error utilizes the count-down method in which a clock generates signals while the shutter is open. These signals are counted and the shutter closure mechanism is prevented from closing if a number of repetitions is exceeded and until an additional number of clock signals are additionally counted. In the above example if the count-down trigger were set at 50 cycles wherein one repetition equals one millisecond of shutter open-time, a ten millisecond exposure would not have time added since the counting mechanism would receive only ten signals during aperature opening. However, at any time over 50 milliseconds an additional number of counts, for example five, could be added to the shutter opening time to keep the shutter open for 55 milliseconds if 50 milliseconds were set. A problem results when 100 millisecond time is set, for example, and the count-down apparatus is not completely responsive to the increased number of counts required. In this example, the device would keep the shutter open an additional five milliseconds for a total of 105 which is probably inadequate for the additional doubling of initially required reciprocity error correction. The nature of the film is such that reciprocity error is negligible below a given exposure time. Above that time, the reciprocity error increases with the length of time the shutter is open.
Accordingly, it is an object of the present invention to provide a film reciprocity error correction circuit which introduces error correction only at the lower light levels where reciprocity is a significant problem.
It is a further object of the present invention to provide a gain modifying circuit for a shutter timing mechanism which compensates for film reciprocity error under those conditions where reciprocity is a significant problem.
A yet further objective of the present invention is to provide a film reciprocity correction circuit which is temperature compensated over a wide range of temperatures to which a camera is normally exposed.